api: add 'Since' tags

These are not actively used by gdbus-codegen or gtk-doc, but they're
helpful anyway so that users know when a given API method was
introduced.

introspection/org.freedesktop.ModemManager1.Modem.Signal.xml

@@ -26,6 +26,8 @@
         @rate: refresh rate to set, in seconds. 0 to disable retrieval.
 
         Setup extended signal quality information retrieval.
+
+        Since: 1.2
     -->
     <method name="Setup">
       <arg name="rate" type="u" direction="in" />
@@ -36,6 +38,8 @@
 
         Refresh rate for the extended signal quality information updates,
         in seconds. A value of 0 disables the retrieval of the values.
+
+        Since: 1.2
     -->
     <property name="Rate" type="u" access="read" />
 
@@ -66,6 +70,8 @@
           </listitem>
         </varlistentry>
         </variablelist>
+
+        Since: 1.2
     -->
     <property name="Cdma" type="a{sv}" access="read" />
 
@@ -112,6 +118,8 @@
           </listitem>
         </varlistentry>
         </variablelist>
+
+        Since: 1.2
     -->
     <property name="Evdo" type="a{sv}" access="read" />
 
@@ -134,6 +142,8 @@
           </listitem>
         </varlistentry>
         </variablelist>
+
+        Since: 1.2
     -->
     <property name="Gsm" type="a{sv}" access="read" />
 
@@ -172,6 +182,8 @@
           </listitem>
         </varlistentry>
         </variablelist>
+
+        Since: 1.2
     -->
     <property name="Umts" type="a{sv}" access="read" />
 
@@ -218,6 +230,8 @@
           </listitem>
         </varlistentry>
         </variablelist>
+
+        Since: 1.2
     -->
     <property name="Lte" type="a{sv}" access="read" />
 
@@ -256,6 +270,8 @@
           </listitem>
         </varlistentry>
         </variablelist>
+
+        Since: 1.16
     -->
     <property name="Nr5g" type="a{sv}" access="read" />